Steam reforming is widely practised and is used to produce hydrogen streams and synthesis gas comprising hydrogen and carbon oxides for a number of processes such as ammonia and methanol synthesis and the Fischer-Tropsch process. Steam reforming may be performed over one or more stages, for example a hydrocarbon feedstock may be reacted with steam over a steam reforming catalyst in pre-reforming or primary reforming steps, followed by partial oxidation of the pre-reformed or partially reformed gas with an oxygen-containing gas, and the resulting gas stream then brought towards equilibrium over a steam reforming catalyst. The water-gas shift reaction also occurs. The reactions may be depicted as follows;
Steam/pre-reformingCxHy + xH2O ⇄ xCO + (y/2 + x)H2Partial OxidationCxHy + x/2 O2 → xCO + y/2 H2CxHy + xO2 → xCO2 + y/2 H2Water-Gas ShiftCO + H2O ⇄ CO2 + H2
In order to obtain a synthesis gas more suited to a Fischer-Tropsch process, a primary or pre-reformed gas is typically subjected to secondary or autothermal reforming in a reformer by partially combusting the primary or pre-reformed gas using a suitable oxidant, e.g. air, oxygen or oxygen-enriched air in a burner apparatus mounted usually near the top of the reformer. The partial oxidation reactions are exothermic and the partial oxidation increases the temperature of the reformed gas to between 1200 and 1500° C. The partially combusted reformed gas is then passed adiabatically through a bed of a steam reforming catalyst disposed below the burner apparatus, to bring the gas composition towards equilibrium. Heat for the endothermic steam reforming reaction is supplied by the hot, partially combusted reformed gas. As the partially combusted reformed gas contacts the steam reforming catalyst, it is cooled by the endothermic steam reforming reaction to temperatures in the range 900-1100° C.
Typically the steam reforming catalyst in the secondary or autothermal reformer is a nickel catalyst supported on alumina or a magnesium-, or calcium-aluminate spinel, but precious metal catalysts can be used. For example EP 0206535 describes a secondary reforming process using a catalytically active metal from Group VIII of the Periodic Table, especially rhodium, wherein the catalyst support is a high purity alumina honeycomb structure.
EP-B-0625481 describes a steam reforming process in an autothermal reformer where, in order to prevent volatilised alumina refractory lining from the combustion zone in a reformer from depositing on the top surface of the steam reforming catalyst, the steam reforming catalyst comprises an upper layer and a lower layer, said upper layer having catalyst particles of reduced activity for the steam reforming reaction. Because of the reduced activity of the upper layer, it will be hotter than the lower layer, therefore preventing deposition of the volatilised refractory in the upper layer of the catalyst bed.